Webclient: extend REST API (#315).

Core API:

The Type interface has a new method:
{{{
boolean hasAssocDef(String childTypeUri)
}}}

Webclient REST API:

A new service call requests a topic's related topics excluding the "property topics":
{{{
GET /webclient/topic/{id}/related_topics
}}}

"Property topics" are child topics that adhere to a parent topic's "Composition" definition.

In preparation of suppressing redundant topics in the detail panel's related topics listing (see #315).